The more & more I stare at my tank looking at the crab who rarely ventures from his hide hole in the rock he really resembles this one:
So not an Emerald Crab at all, I will try again to get a picture of him but it is very hard. He hides almost immediatley when approaching the tank.
The reason I think he is a Stone is because of the coloration of the front claws, I can see the have black tips. It is also very fast and can jump to get food, plus he seems interested in anything that moves as a meal.
Currently no tank inhabitants have died from him but he may have taken a hermit apart and I just didn't notice it yet.
I will update with an actual picture of mine.